An ECG measures the electrical signals emitted by the heart, which are generated by propagation of action potentials that trigger depolarization of heart fibers. Physiologically, transmembrane ionic currents are generated within the heart during cardiac electrical signals from well-established, traditional chest locations. Cardiac depolarization originates high in the right atrium in the sinoatrial (SA) node before spreading leftward towards the left atrium and inferiorly towards the atrioventricular (AV) node. After a delay occasioned by the AV node, the depolarization impulse transits the Bundle of His and moves into the right and left bundle branches as well as Purkinje fibers to activate the right and left ventricles.
During each cardiac cycle, the ionic currents create an electrical field in and around the heart, which can be detected by ECG electrodes placed on the skin over the anterior thoracic region of the patient's body to the lower right and to the lower left of the sternum on the left anterior chest and on the limbs. Cardiac electrical activity is then visually represented in an ECG trace by PQRSTU-waveforms, which can be interpreted post-ECG recordation to derive heart rate and physiology. The P-wave represents atrial electrical activity, and the QRSTU components represent ventricular electrical activity. Specifically, a P-wave represents atrial depolarization, which causes atrial contraction.
P-wave analysis based on ECG monitoring is critical to accurate cardiac rhythm diagnosis and focuses on localizing the sites of origin and pathways underlying arrhythmic conditions. Certain arrhythmias can cause a clinical problem referred to as syncope. Syncope, or a transient loss of consciousness with spontaneous recovery, is often caused by a dramatic drop in blood pressure that leads to a loss of consciousness due to cerebral hypofusion. Conditions that produce cardiac-based syncope are often serious, may be harbingers of sudden death, and can require serious therapy. Such conditions include high-grade AV block, which can lead to an abrupt loss of consciousness and is diagnosed based on the relative position and temporal association of the P-wave with the QRS-wave. Transient ventricular tachycardia is another such condition that can result in syncope where there is a rapid ventricular response (with a series of rapid QRS signals) that is disassociated from atrial activity or the P-wave. However, not all episodes of high-grade AV block or ventricular tachycardia result in syncope; some superficially similar arrhythmias are better tolerated than other arrhythmias, even in the same patient. Often to know from the specific type of arrhythmia alone whether or not syncope will occur is not possible.
Cardiac rhythm disorders are often sporadic and may not occur in-clinic during a conventional 12-second ECG. Syncope episodes can be especially sporadic and infrequent; further, these episodes are problematic because they are common, costly, often disabling, may cause injury, and may be the only warning sign prior to sudden cardiac death (SCD). Establishing the underlying cause of these episodes is important because the cause greatly influences the treatment and prognosis. Cardiac-based syncope portends the highest mortality, in contrast to neurally mediated syncope, such as vasodepressor syncope, or where the patient presents with an apparent syncope episode that is not true syncope, but is due to an alternative physiological condition, including metabolic conditions, such as hypoglycemia; neurological causes, such as seizures; and psychiatric disorders. Moreover, syncope can arise slowly or abruptly. A patient who is gradually aware that he or she may lose consciousness is less prone to injury. However, an abrupt loss of consciousness is much more dangerous because a person with abrupt syncope may be standing and fall suddenly, leading to injury. Therefore, knowing whether or not a condition will likely cause a fall is valuable in properly managing patients, regardless of the basis.
The diagnosis, prognosis, and treatment of syncope can be improved through concomitant recording of syncope episodes as well as ECG data. Where syncope episodes are based on a cardiac condition, diagnosis and treatment are especially important due to the higher mortality rate correlated with cardiac-based syncope episodes. Moreover, both cardiac-based and neurally mediated syncope episodes may require a different treatments. For cardiac arrhythmia-induced syncope, a pace-maker might be helpful, whereas for neurally mediated syncope, a specific drug therapy may be helpful. Correlating motor activity and cardiovascular ECG data is important for improving diagnostic specificity as well as guiding therapy. Moreover, such combined sensor technology can optimize and improve monitoring recommendations during recovery or rehabilitation programs.
Further, combining an ECG recorder with a syncope detection mechanism that detects a sudden collapse is especially valuable because the combination can aid in including (or excluding) a basis for cardiac arrhythmia and inform the doctor on the seriousness of a condition where the recorder can identify falls due to arrhythmia. Continuous ECG monitoring with P-wave-centric action potential acquisition over an extended time period is more likely to elucidate sporadic cardiac events that can be specifically identified and diagnosed, including cardiac events that produce syncope. A longer monitoring period enhances the likelihood of diagnosing an episodic arrhythmia responsible for an episodic syncope episode. However, recording sufficient ECG and physiological data continuously over an extended time period to both diagnose an arrhythmia and syncope that produces a fall remains a technical challenge on multiple levels: cost, comfort, reliability, and both rhythm as well as fall accuracy.
An example of this technical challenge can be seen with actigraphy sensors, such as accelerometers, which can be used to detect movements that occur during syncope episodes, such as falls and sudden postural changes that a patient may experience during a syncope episode while sitting down. Different kinds of actigraphs exist. For example, sleep actigraphs are typically worn similar to a watch on the wrist of the non-dominant arm and can be worn for weeks. Activity actigraphs are worn and used similar to a pedometer, around the waist and near the hip; they are useful in determining the level of activity as well as, potentially, calories and can be worn for a number of days. Movement actigraphs are typically larger and are worn on the shoulder of the dominant arm. Further, movement actigraphs include 3-D actigraphs, which are distinct from 1-D actigraphs that are used during sleep as well as activity actigraphy and tend to include a high sample rate as well as large memory; thus, they are often only used for a few hours. However, wearing two separate devices, one for ECG and another for collecting actigraphy to detect syncope episodes, creates problems. For example, recordings from two separate devices are not synchronized, which could result in temporally mismatched movements indicative of syncope and ECG data.
Current combined actigraphy and ECG monitors are similarly lacking in meeting the technical challenge. For example, U.S. Pat. No. 8,460,189, to Libbus et al. (“Libbus”) discloses an adherent wearable cardiac monitor that includes at least two measurement electrodes and an accelerometer. The device includes a reusable electronics module and a disposable adherent patch, which includes the electrodes. ECG monitoring can be conducted using multiple disposable patches adhered to different locations on the patient's body. The device includes a processor configured to control data collection and transmission from the ECG circuitry, including generating and processing ECG signals as well as data acquired from two or more electrodes. The ECG circuitry and electrodes can be coupled in multiple ways to define an ECG vector; further, the ECG vector orientation can be determined in response to the measuring electrodes' polarity and the orientation of the electrode measurement axis. The accelerometer can be used to determine the orientation of the measuring electrodes at each location. The ECG signals measured at different locations can be rotated based on the accelerometer data to modify the amplitude and direction of the ECG features to approximate a standard ECG vector. The signals recorded at different locations can be combined by summing a scaled version of each signal. Libbus further discloses that inner ECG electrodes may be positioned near outer electrodes to increase the voltage of the measured ECG signals. However, Libbus treats ECG signal acquisition as measuring a simple aggregate directional data signal without differentiating between the distinct types of cardiac electrical activities presented by an ECG waveform, particularly atrial (P-wave) activity. Further, Libbus does not address using the accelerometer data to identify movements that could be indicative of syncope.
Similarly lacking is the SOMNOwatch™ manufactured by SOMNOmedics, a wearable watch-shaped monitoring device that records sleep actigraphy, recognizes sleep/wake rhythms, records activity actigraphs, aids attention deficit and hyperactivity disorder (ADHD) diagnosis, reads ECGs, and records heart rate. The SOMNOwatch™ can only store raw data from a single channel ECG for up to 18 hours, requires a software system that synchronizes heart rate with motor activity, and does not record specific movements, limiting the usefulness of the device for detecting syncope episodes through recognition of movements indicative of syncope, such as falls and sudden postural changes.
Therefore, a need remains for a low-cost, extended-wear, continuously recording ECG monitor coupled with a syncope sensor attuned to detecting low-amplitude cardiac action potential propagation for arrhythmia diagnosis, particularly through atrial activation P-waves, that is practicably feasible for long-term wear and correlating cardiovascular events with movements indicative of syncope or a loss of consciousness.